1. Field of the Invention
The invention relates generally to chucks for holding a wafer and, more specifically, to an improved chuck for clamping large diameter (300 mm or more) wafers containing a mechanical clamp for clamping the periphery of the wafer and an electrostatic clamp for clamping the center of the wafer.
2. Description of the Background Art
Chucks, either mechanical or electrostatic, are used for holding a workpiece in various applications ranging from holding a sheet of paper in a computer graphics plotter to holding a wafer within a semiconductor wafer process chamber. Mechanical chucks typically secure a workpiece, i.e., semiconductor wafer, to the chuck by applying a physical holding force to a clamping ring or calipers located at the periphery of the wafer. The wafer is held in place until the physical force is reversed and the clamping ring or calipers retract. Electrostatic chucks perform this task by creating an electrostatic attractive force between the wafer and the chuck. A voltage is applied to one or more electrodes in the chuck so as to induce opposite polarity charges in the wafer and electrodes, respectively. The opposite charges pull the wafer against the chuck, thereby retaining the wafer.
A diameter of 200 mm is an accepted industry standard for semiconductor wafer size. In semiconductor wafer processing equipment, either type of chuck (mechanical or electrostatic) is used for clamping 200 mm wafers to a pedestal during processing. For example, in a physical vapor deposition (PVD) chamber, a 200 mm wafer is mechanically clamped to the pedestal to ensure that the wafer is stationary during processing. To enhance some PVD processes, the pedestal is heated to heat the wafer to an appropriate temperature to facilitate efficient processing. Increased demand for 200 mm wafers led to improvements in chuck construction and features for processing this size substrate. This resulted in higher yields, better temperature control and an overall better quality product.
The latest generation of semiconductor wafers have diameters of 300 mm to accommodate fabrication of even more integrated circuit components on a single wafer. Unfortunately, the larger size wafers carry with them their own set of production problems. For example, early test designs of chucks for such wafers allow bowing at the wafer center when a backside gas is introduced into the region under the wafer and above the pedestal. Pumping a backside gas into this region is a popular and highly efficient method for transferring heat from the wafer being processed to the pedestal or vice versa. Smaller wafers bow slightly at their center due to the pressure exerted by the gas. Usually, for 200 mm wafers, the bowing is not significant and temperature uniformity across the wafer does not vary greatly. However, for 300 mm wafers, wafer bowing can be substantial. Such bowing interferes with wafer processing and can lead to wafer damage.
For example, FIG. 1 depicts a conventional PVD chamber 100 for processing 300 mm semiconductor wafers. The wafer 102 rests on support surface 105 of a pedestal 104. The pedestal 104 is supported by a pedestal base 106 which contains the necessary wiring to conduct power from a remote power source (not shown) to the heating elements (not shown) within the pedestal 104. The wafer 102 is mechanically clamped to the pedestal 104 by a peripheral clamping ring 108. Lift pins 110 mounted on a platform 112 connected to a vertical shaft 114 serve to lift the wafer 102 off the pedestal surface 105 after processing. Once a wafer 102 is clamped, a heat transfer gas is pumped into the region 113 between the wafer 102 and support surface 105 until an optimal pressure for heat transfer is reached. The bowing of the wafer resulting from the backside gas pressure is seen in the circled portion of FIG. 1. This condition causes a loss of contact area between the wafer and the pedestal as well as non-uniform distribution of heat transfer gas. This in turn contributes to a non-uniform temperature profile across the wafer and may also lead to stress fractures in the substrate material. Maintaining a uniform temperature across the wafer is necessary and important to ensure high performance and yield of the end product. Consequently, processing a 300 mm wafer under these conditions may lead to process irregularities and wafer damage.
Therefore, there is a need in the art for an improved chuck that reduces wafer bowing at the center and improves temperature uniformity across the wafer.